1. Field of the Invention
The present invention relates to a liquid crystal display (LCD) device, and more particularly, to a thin film transistor and a liquid crystal display (LCD) device having the same, to prevent the degradation of thin film transistor when maintaining a uniform width to length (W/L) ratio of the channel across the entire thin film transistor.
2. Discussion of the Related Art
The demand for various display devices has increased with the development of information society. Accordingly, many efforts have been made to research and develop various flat panel display devices such as liquid crystal displays (LCD), plasma display panels (PDP), electroluminescent displays (ELD), and vacuum fluorescent displays (VFD). Some species of flat panel display devices have already been used as displays in various equipment.
Among the various flat panel display devices, liquid crystal display (LCD) devices have been most widely used due to their advantageous characteristics including a thin profile, light weight, and low power consumption, whereby the LCD devices provide a substitute for the traditional cathode ray tube (CRT) display. In addition to mobile LCD devices such as notebook computer displays, LCD devices have been developed for computer monitors and also televisions to receive and display broadcasting signals.
Despite various technical advances in LCD technology, research in enhancing the picture quality of the LCD device has been, in some respects, lacking as compared to advances in other features and advantages of LCD devices. In order to use LCD devices in various environments as a general purpose display, such applications of LCD devices depends on whether LCD devices can implement a high quality picture, such as high resolution and high luminance with a large-sized screen, while still maintaining lightness in weight, thin profile, and low power consumption.
Generally as shown in FIG. 1, a related art LCD device includes upper and lower substrates 10 and 20 bonded to each other with a predetermined gap in between, and a liquid crystal layer (not shown) formed between the upper and lower substrates. At this time, each of the upper and lower substrates 10 and 20 includes a display area for displaying image signals, and a non-display area.
On the display area of the lower substrate 20, there are a plurality of gate lines arranged at fixed intervals, a plurality of data lines arranged substantially perpendicular to the gate lines to define pixel regions, and a plurality of pixel electrodes respectively formed in the pixel regions of a matrix defined by the gate and data lines. In addition, a plurality of thin film transistors TFT are formed at an area where of the gate and data lines cross, thereby applying data signals to the respective pixel electrodes according to gate signals.
The display area of the upper substrate 10 has a color filter layer corresponding to the pixel regions of the lower substrate 20, a black matrix layer corresponding to other portions except the pixel regions, and a common electrode formed on an entire surface.
As signals are applied to the gate lines in sequence, the data signal is applied to the pixel electrode of the corresponding line, thereby displaying images.
Generally, an active matrix LCD device which is used for a large-sized and high-resolution display device includes a thin film transistor in each pixel to drive each pixel in an LCD panel and a driving circuit. The driving circuit drives the thin film transistor to control each pixel and applies driving signals to gate and data lines.
Driving circuits may be classified into two types, i.e., an external signal line driving circuit that is directly connected with the LCD panel by forming an additional direct circuit on an external substrate of the LCD panel as well as the thin film transistor for pixel driving and a thin film transistor formed at the same time as the driving circuit.
The thin film transistor formed at the same time as the driving circuit is provided in the non-display area (bezel) of the LCD device. Also, the thin film transistor formed at the same time as the driving circuit is generally formed of a polysilicon thin film transistor (poly-TFT) having a large electron mobility.
Hereinafter, a related art polysilicon thin film transistor will be described as follows.
FIGS. 2 and 3 are plan views showing heat emission paths by widths in a related art polysilicon thin film transistor.
Generally, a polysilicon thin film transistor is driven by a power of Ids×Vds. When the polysilicon thin film transistor is driven, the polysilicon thin film transistor generates heat. However, because the polysilicon thin film transistor has a low heat conductivity, the heat generated in the polysilicon thin film transistor is not easily emitted to the outside.
In a high-power area of the channel, the self-heating effect of the polysilicon thin film transistor becomes serious, thereby causing the degradation of the thin film transistor. Especially, if the channel width is large, the self-heating effect of the polysilicon thin film transistor becomes more serious.
FIG. 2 illustrates the plan view of a polysilicon thin film transistor having a small channel width, and FIG. 3 illustrates the plan view of a polysilicon thin film transistor having a large channel width.
If the self-heating effect is generated in the polysilicon thin film transistor having the small channel width shown of FIG. 2, the heat emission path is short in both X and Y directions, whereby the generated heat is emitted to the outside in a short time.
However, if the self-heating effect is generated in the polysilicon thin film transistor having the large channel width shown of FIG. 3, the heat emission path is long in the X direction. For example, in the case of the heat generated in the center of the channel, the heat emission path of the Y direction is short, and the heat emission path of the X direction is long. Accordingly, the generated heat is left inside the polysilicon thin film transistor, thereby causing the degradation of the polysilicon thin film transistor. As shown above, as the channel width increases, the possibility of degradation in the polysilicon thin film transistor increases due to the residual heat of the self-heating effect.
Hereinafter, the degradation of the polysilicon thin film transistor as a function of the W/L ratio will be explained as follows.
FIGS. 4 and 5 illustrate data showing variation of Δ Vth (threshold voltage) according to an application voltage (Vgs=Vds) and a power density for various W/L ratios of the channel.
FIG. 6 illustrates data showing a power density versus the W/L ratio when the length of channel is 6 μm.
FIG. 7 illustrates data showing the reliability of the thin film transistor versus the W/L ratio of channel.
First, in FIGS. 4 and 5 when a W/L ratio of the channel 120/6, 80/6, 40/6, 20/6 and 6/6(μm/μm), a variation of Δ Vth is shown depending on a power density and an application voltage (Vgs=Vds) of the thin film transistor.
As shown in FIG. 4, if the same voltage (Vgs=Vds) is applied to each of the thin film transistors having the W/L ratios of 120/6, 80/6, 40/6, 20/6 and 6/6(μm/μm), the value of ΔVth increases as the W/L ratio increases.
As shown in FIG. 5, if the same power is applied to each of the thin film transistors having the W/L ratios of 120/6, 80/6, 40/6, 20/6 and 6/6(μm/μm), the value of Δ Vth increases as the W/L ratio increases.
As described in the data of FIGS. 4 and 5, if the channel length is the same, the possibility of degradation increases in the thin film transistor having the large channel width (having the large W/L ratio).
In FIG. 6, as the W/L ratio increases with a value of Δ Vth corresponding to 1V the power density decreases.
Based on the above data, if the channel length (L) is the same, the degradation of thin film transistor caused by the self-heating effect results even with lower power density as increasing the channel width (the large W/L ratio or the large channel area), thereby lowering the reliability.
To prevent the degradation of the thin film transistor caused by the self-heating effect of the polysilicon thin film transistor when forming the thin film transistor having a large channel width, it is advantageous to form a multi-channel type thin film transistor.
If the uniform W/L ratio is maintained for each channel in the multi-channel type thin film transistor, the W/L ratio of each channel decreases and the entire size of thin film transistor increases as the number of channels increases, and the W/L ratio of each channel increases and the entire size of thin film transistor decreases as the number of channels decreases.
When maintaining the uniform W/L ratio in the entire thin film transistor, the reliability of thin film transistor versus W/L ratio of each channel will be explained as follows.
As explained above, if the uniform W/L ratio in the entire thin film transistor is maintained, the number of channels increases as the W/L ratio of each channel decreases. For example, when the W/L ratio of each channel is the largest, a signal channel is provided in the thin film transistor. In the case when the W/L ratio of each channel is large, the interval between each of the channels decreases, so that the entire size of thin film transistor decreases.
Based on the data of FIG. 7, in the case of the thin film transistor having a large W/L ratio of each channel (that is, the entire size of thin film transistor decreases), Δ Vth increases even though the same voltage (Vgs=Vds) is applied.
When the uniform W/L ratio is maintained in the entire thin film transistor, the reliability is lowered as the W/L ratio of unit channel increases.
The related art polysilicon thin film transistor has the following disadvantages.
First, the degradation of thin film transistor becomes serious due to the self-heating effect as the W/L ratio in the thin film transistor increases.
Also, when applying the same driving voltage, the reliability degrades as the W/L ratio increases.
In addition, even though the uniform W/L ratio is maintained in the thin film transistor, the W/L ratio of each channel increases and the entire size of thin film transistor decreases, thereby lowering the reliability of thin film transistor.